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Topic: potentiometer voltage reduction (Read 395 times) previous topic - next topic

Noobian

Hi,

I have a 1.5v dc motor and I want to power it using a 3v wall adapter with rating of 500ma

I do not the the resistor value to reduce 3v to 1.5v so I'm thinking about using a 1k ohm potentiometer
which I have lying around.

is it a good option, since I need the motor running 24/7, will the pot burn out, or should I use a regular buck converter?

please advice,
thanks in advance

MarkT

No its a very bad option.  You will simply burn out the pot.

Voltage dividers are not used to reduce voltage of power supplies, only signals, since
from a power perspective they are completely inefficient.

You reduce voltage for power to a motor with PWM.   Alternatively a buck converter
capable of the motor's maximum current draw could be used.  500mA is unlikely to
be adequate.

If you want a small electric motor to run 24/7 you will need either a quality brushed
motor or brushless.  Cheap small electric motors wear out quite quickly.

Perhaps if you asked what you wanted to achieve you will find out a better approach -
this could be the xy problem again.  http://xyproblem.info/
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Noobian

No its a very bad option.  You will simply burn out the pot.

Voltage dividers are not used to reduce voltage of power supplies, only signals, since
from a power perspective they are completely inefficient.

You reduce voltage for power to a motor with PWM.   Alternatively a buck converter
capable of the motor's maximum current draw could be used.  500mA is unlikely to
be adequate.

If you want a small electric motor to run 24/7 you will need either a quality brushed
motor or brushless.  Cheap small electric motors wear out quite quickly.

Perhaps if you asked what you wanted to achieve you will find out a better approach -
this could be the xy problem again.  http://xyproblem.info/
If you want me to be more precise then here goes, I have a 1.5v motor which i've been using to pump air into my aquarium, I've been running it with a 3v 400ma solar panel since the last 6 months, now that I have decided i want it to run 24/7 instead of only during the day so I wanted to power it through a wall brick adapter, the smallest value I have is 3v 500ma adapter, i dont have a buck converter or know resistor value to reduce the voltage to the desired one. so i was thinking maybe I can use a pot instead.

Now that you've told me that the pot will burn out because thats not what its actually used for, it helped.

Is there anyway you can tell me the resistor value which i should use?

thanks.

Delta_G

The resistor would be any.  It's a resistor divider, as long as the two resistors are the same the voltage in between them will be half.

But that's ONLY if you're not pulling power.  As soon as you start running your motor the voltage is going to sag. 

Resistor dividers are good for getting control voltages or measuring voltages out of range, but you can't use them to source power for something.  That's not only going to waste a ton of power, it won't run your motor right either. 

You either need to invest in a suitable power supply or a buck converter to use the one you have.  You can't get there with a resistor. 
|| | ||| | || | ||  ~Woodstock

Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

MorganS

If you have a 1.5v motor and a 3v supply then a resistor in series with the motor should be the same value as the motor's resistance. However this is hard to measure since the motor resistance (impedance actually) varies with speed. It also needs to be the same power as the motor (1.5 watt) as it will dissipate the same power as heat. This is actually a big power rating for a resistor.
"The problem is in the code you didn't post."

CrossRoads

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

DVDdoug

Quote
using a 3v wall adapter with rating of 500ma
Is that power supply regulated?   Or, do you have a multimeter to confirm it's stable and reliable?     An unregulated power supply can vary quite a bit from it's "nominal" voltage.

A quick-and-dirty solution would be two silicon diodes in series with the motor.   The diodes will drop ~0.6 - 0.7V each and that should be close enough for a 1.5V motor.   The 1N400x is cheap and can handle up to 1 Amp.

As with a resistor or any linear regulator or "voltage drop" solution, the 2 diodes together will dissipate about the same amount of heat/energy as the motor.   Since the same current flows through both and about the the same voltage is applied/dropped across both.

MorganS

If your primary source is Sparkfun, Adafruit, Radio Shack or similar, then 1.5W is an unusual resistor.

It's not rare but it is obviously bigger than the regular 1/4W resistors that are widely available.
"The problem is in the code you didn't post."

CrossRoads

That could be.  I use Digikey and Mouser as my primary sources of components.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Paul__B

Use the two silicon power diodes.

MarkT

Why not get a 3V pump?  The problem with a ballast resistor in series is the motor will still run fast
with no load, and will stall much more readily under load - a constant voltage helps regulate motor
speed.  Or a 1.65V regulator? http://www.micrel.com/_PDF/mic3975.pdf
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Noobian

Use the two silicon power diodes.

Is that power supply regulated?   Or, do you have a multimeter to confirm it's stable and reliable?     An unregulated power supply can vary quite a bit from it's "nominal" voltage.

A quick-and-dirty solution would be two silicon diodes in series with the motor.   The diodes will drop ~0.6 - 0.7V each and that should be close enough for a 1.5V motor.   The 1N400x is cheap and can handle up to 1 Amp.

As with a resistor or any linear regulator or "voltage drop" solution, the 2 diodes together will dissipate about the same amount of heat/energy as the motor.   Since the same current flows through both and about the the same voltage is applied/dropped across both.
ok, i think i will use the diodes

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